A light fixture comprising an array of differing color light emitting diodes (LEDs) can be used, with appropriate control, to generate a continuous range of colors of illumination. The brightness of each of the colors of LEDs in the light fixture may be controlled by modulating the drive current with which the LEDs are supplied, and the lifetime of the LED can be maximized if such drive currents are uniform over time.
Many examples exist of control circuits exploiting pulse width modulation (PWM) control of the average time that the LEDs are connected to a voltage source. In these cases, it is assumed that the current-voltage characteristic of the LEDs will remain constant over time, so that the peak current through the LEDs will be constant for a constant voltage source. The average current is then a function of the PWM fraction with which the LEDs are driven. In such schemes, the actual brightness of the LEDs will vary as their characteristics vary, with temperature and age, for example, and the PWM fraction required for a given brightness will vary between LEDs and with the number of LEDs driven. However, these schemes have an advantage in that only a single voltage source is required for all the LEDs in the array.
For example, in published international PCT patent application no. PCT/US01/50156 (WO 02/061330), methods and apparatus for illuminating liquids are described. In one described example, multicolor LED light sources are employed to achieve a wide range of enhanced lighting effects in liquids; such liquids include water in pool or spa environments. In another example, a pool or spa is illuminated by one or more multicolored light sources that may be employed as individually and independently controllable devices, or coupled together to form a networked lighting system to provide a variety of programmable and/or coordinated color illumination effect in the pool or spa.
Moreover, in U.S. Pat. No. 6,016,038, LED systems capable of generating light for illumination and display purposes, and methods of operating such systems are described. The LEDs are capable of being controlled by a processor to alter the brightness and/or color of light radiation emitted therefrom, such control using PWM signals. Thus, illumination from the LED systems is susceptible to being controlled by a computer program to provide complex, pre-designed patterns of light in virtually any environment. U.S. Pat. No. 6,150,774 is a further example of a PWM-based implementation of a LED lighting system.
However, PWM control of LEDs is not always technically appropriate and alternative approaches to conventional PWM control may capable of providing at least one of lower manufacturing cost, more efficient power conversation when energizing LEDs, or greater physical compactness. In order to overcome the problem of variation in brightness of LEDs driven from a constant voltage source with PWM control, it is possible to exploit the use of current mode control of the LED brightness. This current mode control can be achieved either by introducing a fixed current limit to each PWM pulse or by using a variable current source for each group of LEDs to be controlled. In the former case, the LEDs are driven with a discontinuous waveform, thereby comprising their lifetime for a given brightness; in the latter, each variable current source entails significant extra cost.
LED-based lights can also be powered from a wide variety of supplies. Further, it is of benefit if a single control circuit can be used for a wide variety of LED array configurations.
LEDs are inherently more efficient than incandescent light sources. In applications where the light fixture is to be mounted in confined spaces, this can be a considerable advantage as less waste heat is lost. In these applications, the efficiency of the controller circuit is also important. For example, circuits employing switch-mode circuit techniques offer considerably higher efficiencies for power conversion and current regulation than linear equivalents. A LED driver circuit is disclosed in U.S. Pat. No. 5,736,881. The circuit disclosed in this patent includes a quasi-resonant circuit as its constant current source.
The present invention affords an improved power controller circuit that is especially appropriate, but not limited to, controlling power delivered to LEDs to modulate their brightness.